While organic electroluminescent (EL) devices have been known for over two decades, their performance limitations have represented a barrier to many desirable applications. In simplest form, an organic EL device is comprised of an anode for hole injection, a cathode for electron injection, and an organic medium sandwiched between these electrodes to support charge recombination that yields emission of light. These devices are also commonly referred to as organic light-emitting diodes, or OLEDs. Representative of earlier organic EL devices are Gurnee et al. U.S. Pat. No. 3,172,862, issued Mar. 9, 1965; Gurnee U.S. Pat. No. 3,173,050, issued Mar. 9, 1965; Dresner, “Double Injection Electroluminescence in Anthracene”, RCA Review, Vol. 30, pp. 322-334, 1969; and Dresner U.S. Pat. No. 3,710,167, issued Jan. 9, 1973. The organic layers in these devices, usually composed of a polycyclic aromatic hydrocarbon, were very thick (much greater than 1 μm). Consequently, operating voltages were very high, often >100V.
More recent organic EL devices include an organic EL element consisting of extremely thin layers (e.g. <1.0 μm ) between the anode and the cathode. Herein, the term “organic EL element” encompasses the layers between the anode and cathode. Reducing the thickness lowered the resistance of the organic layer and has enabled devices that operate much lower voltage. In a basic two-layer EL device structure, described first in U.S. Pat. No. 4,356,429, one organic layer of the EL element adjacent to the anode is specifically chosen to transport holes, and therefore, it is referred to as the hole-transporting layer, and the other organic layer is specifically chosen to transport electrons, and is referred to as the electron-transporting layer. Recombination of the injected holes and electrons within the organic EL element results in efficient electroluminescence.
There have also been proposed three-layer organic EL devices that contain an organic light-emitting layer (LEL) between the hole-transporting layer and electron-transporting layer, such as that disclosed by Tang et al [J. Applied Physics, Vol. 65, Pages 3610-3616, 1989]. The light-emitting layer commonly consists of a host material doped with a guest material, also known as the dopant. Still further, there has been proposed in U.S. Pat. No. 4,769,292 a four-layer EL element comprising a hole-injecting layer (HIL), a hole-transporting layer (HTL), a light-emitting layer (LEL) and an electron transport/injection layer (ETL). These structures have resulted in improved device efficiency.
In organic electroluminescent devices, only 25% of electrons and holes recombine as singlet states while 75% recombine as triplet states according to simple spin statistics. Singlet and triplet states, and fluorescence, phosphorescence, and intersystem crossing are discussed in J. G. Calvert and J. N. Pitts, Jr., Photochemistry (Wiley, N.Y., 1966) and further discussed in publications by S. R. Forrest and coworkers such as M. A. Baldo, D. F. O'Brien, M. E. Thompson, and S. R. Forrest, Phys. Rev. B, 60, 14422 (1999). The singular term “triplet state” is often used to refer to a set of three electronically excited states of spin 1 that have nearly identical electronic structure and nearly identical energy and differ primarily in the orientation of the net magnetic moment of each state. A molecule typically has many such triplet states with widely differing energies. As used hereinafter, the term “triplet state” of a molecule will refer specifically to the set of three spin-1 excited states with the lowest energy, and the term “triplet energy” will refer to the energy of these states relative to the energy of the ground state of the molecule. Similarly, the term “singlet energy” will refer to the energy of the lowest excited singlet state relative to that of the ground state of the molecule. Emission from triplet states is generally very weak for most organic compounds because the transition from triplet excited state to singlet ground state is spin-forbidden. Hence, many emitting materials that have been described as useful in an OLED device emit light from their excited singlet state by fluorescence and thereby utilize only 25% of the electron and hole recombinations. Thus, it is possible, by the proper choice of host and dopant, to collect energy from both the singlet and triplet excitons created in an OLED device and to produce a very efficient phosphorescent emission. The term electrophosphorescence is sometimes used to denote electroluminescence wherein the mechanism of luminescence is phosphorescence. However, it is possible for compounds with states possessing a strong spin-orbit coupling interaction to emit strongly from triplet excited states to the singlet ground state (phosphorescence). One such strongly phosphorescent compound is fac-tris(2-phenyl-pyridinato-N^C—)Iridium(III) (Ir(ppy)3) that emits green light (K. A. King, P. J. Spellane, and R. J. Watts, J. Am. Chem. Soc., 107, 1431 (1985), M. G. Colombo, T. C. Brunold, T. Reidener, H. U. Güdel, M. Fortsch, and H.-B. Bürgi, Inorg. Chem., 33, 545 (1994)). Organic electroluminescent devices having high efficiency have been demonstrated with Ir(ppy)3 as the phosphorescent material and 4,4′-N,N′-dicarbazole-biphenyl (CBP) as the host (M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson, S. R. Forrest, Appl. Phys. Lett., 75, 4 (1999), T. Tsutsui, M.-J. Yang, M. Yahiro, K. Nakamura, T. Watanabe, T. Tsuji, Y. Fukuda, T. Wakimoto, S. Miyaguchi, Jpn. J. Appl. Phys., 38, L1502 (1999)). Additional disclosures of phosphorescent materials and organic electroluminescent devices employing these materials are found in U.S. Pat. No. 6,303238 B1, WO 00/57676, WO 00/70655 and WO 01/41512 A1, Yersin et al, Proceedings of SPIE Vol. 5214, 124-132.
Bryan et al U.S. Pat. No. 5,141,671 disclose mixed-ligand aluminum chelate complexes having the property of blue emission for use in organic electroluminescent devices. Tsuji et al US 2003/0129452 A1 disclose the use of these blue-emissive aluminum chelate compounds as single host materials in red phosphorescent organic electroluminescent devices. Seo US 2002 0101154 A1 discloses a prophetic example of a device with a light emitting layer comprising a particular one of the blue-emissive aluminum chelates, bis(2-methyl-8-quinolinolato)(4-phenyl-phenolato)aluminum(III), and NPB and the PtOEP dopant in a composition of 80:20:4, respectively.
In the absence of experimental data the triplet energies may be estimated in the following manner. The triplet state energy for a molecule is defined as the difference between the ground state energy (E(gs)) of the molecule and the energy of the lowest triplet state (E(ts)) of the molecule, both given in eV. These energies can be calculated using the B3LYP method as implemented in the Gaussian98 (Gaussian, Inc., Pittsburgh, Pa.) computer program. The basis set for use with the B3LYP method is defined as follows: MIDI! for all atoms for which MIDI! is defined, 6-31G* for all atoms defined in 6-31G* but not in MIDI!, and either the LACV3P or the LANL2DZ basis set and pseudopotential for atoms not defined in MIDI! or 6-31G*, with LACV3P being the prefer-red method. For any remaining atoms, any published basis set and pseudopotential may be used. MIDI!, 6-31 G* and LANL2DZ are used as implemented in the Gaussian98 computer code and LACV3P is used as implemented in the Jaguar 4.1 (Schrodinger, Inc., Portland Oreg.) computer code. The energy of each state is computed at the minimum-energy geometry for that state. The difference in energy between the two states is further modified by Equation 1 to give the triplet state energy (E(t)):E(t)=0.84*(E(ts)−E(gs))+0.35   (1)
For polymeric or oligomeric materials, it is sufficient to compute the triplet energy over a monomer or oligomer of sufficient size so that additional units do not substantially change the computed triplet energy of the phosphorescent light emitting material.
One class of useful phosphorescent materials is the transition metal complexes having singlet ground states and triplet excited states. For example, fac-tris(2-phenylpyridinato-N,C2′)iridium(III) (Ir(ppy)3) strongly emits green light from a triplet excited state owing to the large spin-orbit coupling of the heavy atom and to the lowest excited state which is a charge transfer state having a Laporte allowed (orbital symmetry) transition to the ground state (K. A. King, P. J. Spellane, and R. J. Watts, J. Am. Chem. Soc., 107, 1431 (1985), M. G. Colombo, T. C. Brunold, T. Reidener, H. U. Güdel, M. Fortsch, and H.-B. Bürgi, Inorg. Chem., 33, 545 (1994). Small-molecule, vacuum-deposited OLEDs having high efficiency have also been demonstrated with Ir(ppy)3 as the phosphorescent material and 4,4′-N,N′-dicarbazole-biphenyl (CBP) as the host (M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson, S. R. Forrest, Appl. Phys. Lett., 75, 4 (1999), T. Tsutsui, M.-J. Yang, M. Yahiro, K. Nakamura, T. Watanabe, T. Tsuji, Y. Fukuda, T. Wakimoto, S. Miyaguchi, Jpn. J. Appl. Phys., 38, L1502 (1999)). LeCloux et al. in International Patent Application WO 03/040256 A2, and Petrov et al. in International Patent Application WO 02/02714 A2 teach additional iridium complexes for electroluminescent devices.
Additives have been used to improve the efficiency of triplet OLED devices. In U.S. 2002/0071963 A1, 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) was used as an additive to a triplet light-emitting layer. However, DCJTB suffers from having singlet emission in the red region of the spectrum and therefore can affect the color of the emission of a triplet OLED device. In particular, if an OLED device with blue or green emission is desired, a red contribution from DCJTB emission would be undesirable.
Notwithstanding these developments, there remains a need for new host materials, and especially hosts that will function with phosphorescent materials to provide improved efficiency, stability, manufacturability, or spectral characteristics of electroluminescent devices.